1. Field of the Invention
The present invention relates to methods for manufacturing a SrTiO3 series varistor, the method comprising sintering in a reducing atmosphere a SrTiO3 composition having at least one of a donor impurity such as Niobium and an acceptor impurity such as aluminum or iron, and selectively oxidizing grain boundaries present in the composition.
2. Background of the Invention
Strontium Titanium Oxide (SrTiO3) series variable resistors (“varistors”) have higher dielectric constants than previously examined Zinc oxide (ZnO) varistors, and also exhibiting low loss coefficients. It has been shown that SrTiO3 varistors exhibit usable non-linear current-voltage characteristics when the interior grain of the SrTiO3 compositions are conductive and grain boundaries within the SrTiO3 compositions display an electrical conduction barrier (Clark, D. R., J. Am. Ceram. Soc. 82:485 (1999)). The conductivity of the SrTiO3 grain can be improved by doping with donor impurities SrTiO3 followed by sintering the doped SrTiO3 under a reducing atmosphere to make an n-type semiconductor (Gupta. K., J. Am. Ceram. Soc. 73:1871 (1990)). When doped SrTiO3 is sintered under a reducing atmosphere, donor impurities such as Niobium (Nb) are present in the grain (Chung, S.-Y. et al., J. Am. Ceram. Soc. 85: 2805, (2002)), whereas acceptor impurities such as Aluminum (Al) are present on the grain boundary. Grain boundaries present within the doped SrTiO3 compositions can function as acceptor states within the compositions or as non-conducting materials present at the grain boundary or diffused within the solid (Gupta. K., J. Am. Ceram. Soc. 73:1871 (1990)).
A general method for forming an acceptor state on a grain boundary is by applying an oxide such as Lead (PbO), Bismuth (Bi2O3) or Lithium (Li2O3) to the surface of a composition followed by sintering under a mixed atmosphere and heat-treating in air. This process forms a liquid oxide that can infiltrate the grain boundary (see, e.g., Korean Patent Application No. 10-1999-0002445 and U.S. Pat. No. 4,612,160). However, this method is complicated and is difficult to control the diffusion depth of the oxide solute source. Additionally, a diffusion-induced grain boundary migration can be formed during the second heat treatment that results in widening of the grain boundary, and formation of a conductive barrier having a thickness of several microns so that the composition cannot function as a varistor (Jeon, J.-H. and Kang, S.-J. L., J. Am Ceram. Soc. 77:1688 (1994)). Other researchers have demonstrated a method whereby the composition is doped with an acceptor impurity followed by sintering under an oxidizing atmosphere to form a grain that operates as an acceptor state, and which results in non-linear current-voltage characteristics (Korean Patent Application No. 10-1999-0057370). However, this process can result in a non-uniform microstructure comprising large grains (>10 microns (μm) in size) that result from abnormal grain growth during the oxidative sintering process. It has been shown that non-linear current-voltage characteristics of the compositions increase as the grain size is decreased (see Gupta. K., J. Am. Ceram. Soc. 73:1871 (1990)). Thus, the non-uniformity of the microstructure should be controlled and grain growth should be deterred during the manufacturing process.
The grain conductivity of SrTiO3 varistor compositions can be enhanced by sintering under a mixed atmosphere of 95% N2/5% H2 or in air (Korean Patent Application No. 10-1999-0057370; Kutty, T. R. N. and Philip, S., Mater. Sci. Eng. B33:58 (1995); and Kuwabara, M., and Matsuda, H., J. Mater. Sci. 34:2653 (1999)). However, these processes can also result in abnormal grain growth within the SrTiO3 (see Chung, S.-Y. et al., Acta Mater. 50:3361 (2002). What is needed is a method of sintering SrTiO3 compositions in which abnormal growth of the grains is deterred, in order to obtain a SrTiO3 composition having a uniform microstructure and electrical properties.